Much work has been done over the years to convert heavy petroleum feedstocks to lighter and more valuable liquid products. One process developed for accomplishing this conversion is fluid coking. In conventional fluid coking, a heavy petroleum feedstock is injected into a fluidized bed of hot, fine coke particles and is thus distributed uniformly over the surfaces of the coke particles where it is cracked to vapors and coke. The vapors pass through a cyclone which removes most of the entrained coke particles. The vapor is then discharged into a scrubber where the remaining coke particles are removed and the products cooled to condense heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 weight percent coke particles, is recycled to the coking reactor. The overhead products from the scrubber are sent to fractionation for separation into gas, naphtha, and light and heavy gas oils.
The coke particles in the reactor vessel flow downwardly to a stripping zone at the base of the reactor where stripping steam removes interstitial product vapors from, or between, the coke particles, as well as some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser which leads to a burner where sufficient air is injected for burning part of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking reactor where the unburned hot coke is recycled thereto. Net coke, above that consumed in the burner, is withdrawn as product coke.
Another type of fluid coking process employs three vessels: a reactor, a heater, and a gasifier. Coke produced in the reactor is withdrawn, and is passed through the heater where a portion of the volatile matter is removed. The coke is then passed to a gasifier where it reacts, at elevated temperatures, with air and steam to form a mixture of carbon monoxide, carbon dioxide, hydrogen, nitrogen, water vapor, and hydrogen sulfide. The gas produced in the gasifier is heat exchanged in the heater to provide part of the reactor heat requirement. The remainder of the heat is supplied by circulating coke between the gasifier and the heater.
Still another type of fluid coking process is a so-called once-through coking process wherein the bottoms fraction from the scrubber is passed directly to a hydrotreating unit instead of being more conventionally recycled to extinction. The disadvantage with such a once-through process is that the bottoms fraction is so laden with fine coke particles that plugging of the hydrotreating unit occurs.
Hydrotreating, as used herein, refers to a process for upgrading a hydrocarbonaceous oil, below cracking temperatures, in the presence of hydrogen and a hydrotreating catalyst such as those containing one or more Group VIB and one or more Group VIII metals on an alumina, silica, or alumina-silica support. During hydrotreating, undesirable constituents, such as nitgrogen and sulfur, are removed.
The hydrotreated filtrate is then passed to a fluid catalytic cracking unit for producing gasoline fractions. As is well known, the catalytic cracking of petroleum fractions is one of the major refining processes for converting petroleum fractions such as a virgin gas oil boiling between 600.degree. F. and 1050.degree. F., to desirable fuel products, such as heating oils and high octane gasoline. Illustrative of "fluid" catalytic conversion processes is the fluid catalytic cracking process wherein suitably preheated high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated riser reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to affect the desired degree of cracking to lower molecular weight hydrocarbons suitable as gasoline fractions.
A wide variety of petroleum cracking catalysts are described in the literature and are commercially available for use in fluidized cracking processes. Commercial cracking catalysts currently in general use comprise a crystalline aluminosilicate zeolite cracking component in combination with an inorganic oxide matrix component. Typical zeolites combined with the inorganic oxide matrix include hydrogen- and/or rare earth metal-exchanged synthetic faujasite of the X- or Y-type and the like. The matrix generally includes amorphous silica-alumina gel and/or a clay material such as, for example, kaolin.
There still exists a need in the art for improved fluidized coking processes which are not limited by the disadvantages of the prior art and which results in increased throughput, increased yields, or both.